1. Field of the Invention
The present invention relates to an area sensor having a light emitter and a light receiver that have a plurality of optical axes provided with in a detection area, each optical axis connecting a set of a light emitting device and a light receiving device. In particular, the invention relates to an area sensor that permits correct matching of optical axes during installation.
2. Description of the Related Art
An area sensor is a kind of switch that comprises a light emitter including light emitting devices and a light receiver including light receiving devices, a set of one light emitting device and one light receiving device forming an optical axis. If any one of the optical axes is interrupted by a moving object, the area sensor turns on. Having a wide area within which the presence or absence of an object can be detected, the area sensor ensures the safety of the operators of machine tools, punching machines, pressing machines, brakes, molding machines, automatic controllers, winding machines, robots, casting machines and the like. In the case of a pressing machine, the area sensor is positioned in a dangerous zone of the machine which is the detection area and when fingers or any other part of the operator's body enters the detection area and interrupts a particular optical axis, the sensor detects that phenomenon and takes an immediate protective action by shutting down the machine or issuing an alarm.
The area sensor is also used in an automatic production line of a plant, where it detects the presence or absence of a moving article and signals for a transfer to the next step upon detecting the article. In this case, the area sensor works as a sensor for automatic control.
An area sensor of this type is shown in FIG. 8 and it comprises a light emitter 2 in which a plurality of light emitting devices 21 such as light emitting diodes (LEDs) that emit infrared rays or the like are spaced on a specified pitch (in FIG. 8, four light emitting devices are provided) and a light receiver 3 in which a corresponding number of light receiving devices 31 such as phototransistors that are spaced on a specified pitch in correspondence with the light emitting devices 21 so that they receive infrared beams 5 emitted from the light emitting devices 21 in the light emitter 2. The light emitter 2 and the light receiver 3 are provided in a face-to-face relationship such that the emitter 2 is positioned on one side of the detection area where the operator of a pressing machine or the like must be protected whereas the receiver 3 is positioned on the other side of the detection area. Optical beams issued from the light emitting devices 21 in the light emitter 2 travel to the corresponding light receiving devices 31 in the light receiver 3.
A specific configuration of the conventional light emitting devices 21 in the light emitter 2 shown in FIG. 8 and a specific configuration of the conventional light receiving devices 31 in the light receiver 3 also shown in FIG. 8 will now be described with reference to FIGS. 9(a) and 9(b), respectively. As shown in FIG. 9(a), each light emitting device 21 is composed of a light emitting diode 22 and a light emitting lens 23 through which the infrared light from the light emitting diode 22 is emitted as infrared beams in a desired pattern of luminous intensity distribution. In the conventional light emitter 2, the light emitting diodes 22 (which are four in number in FIG. 9(a)) all have identical characteristics. Similarly, the light emitting lenses 23 (which are four in number in FIG. (9)) all have identical characteristics.
As shown in FIG. 9(b), each light receiving device 31 is composed of a light receiving lens 33 that receives and collects the infrared beams emitted from the corresponding light emitting device 21 and a phototransistor 32 that issues an electrical signal upon receiving the collected infrared beams from the lens 33. In the conventional light receiver 3, the phototransistors 32 (which are four in number in FIG. 9(b) all have identical characteristics. Similarly, the light receiving lenses 33 (which four in number in FIG. 9(b) all have identical characteristics. Therefore, the angular characteristics that permit detection are substantially the same with respect to all optical axes involved.
The term "angular characteristics" as used herein refers to a region which, as seen from the center of the lens in a light emitting device or a light receiving device, allows each device to emit or receive light. Since this region assumes a substantially conical shape having the vertex at the stated center, it is called "angular characteristics" with respect to an optical axis.
The light emitter 2 and the light receiver 3 are both controlled with respective built-in control circuits (not shown) such that the emission of light from the light emitting devices 21 in the light emitter 2 and its reception by the corresponding light receiving devices 31 satisfy a specified timed relationship.
In order to ensure that the entrance of an object into the detection area is detected positively, the light emitter 2 and the light receiver 3 must be installed in such a way that the optical axis of each light emitting device 21 matches the optical axis of the corresponding light receiving device 31. To check for any mismatch, an indicator 6 is provided on the side of the light receiver as shown in FIG. 8. If any one of the optical axes is interrupted, the indicator 6 signals in red and only if all optical axes travel unblocked, does the indicator signal in green. Therefore, when an engineer is to install the area sensor, he only has to keep an eye on the indicator 6 while moving the light emitter (or light receiver) relative to the light receiver (or light emitter) and fixes the sensor in the position at which the red signal from the indicator 6 turns green.
FIG. 10(a) illustrates the ideal state of sensor installation in which all optical axes of the light emitter 2 align with those of the light receiver 3 as seen in a direction normal to the plane in which the optical axes lie. In FIG. 10(a), reference numeral 2 designates the light emitter, 3 designates the light receiver, 5 designates infrared beams having specified angular characteristics, and the area delineated by dashed lines indicates a detectable angular range. Suppose here that a light interrupting object 9 (see FIG. 10(b)) such as fingers or some other part of the human body gets into the detection area of the sensor that has been installed in the ideal state shown in FIG. 10(a). Then, the infrared beams 5 striking the object 9 are interrupted and no longer received by the corresponding light receiving device 31 in the light receiver 3, whereupon the light receiver 3 issues an alarm or shuts down the machine to ensure safety for the operator.
If the individual optical axes of the area sensor have wide angular characteristics as shown in FIG. 10(a), axial matching is easy to accomplish during sensor installation and, hence, convenience in use is assured.
However, in this case, even if the optical axes in the light emitter 2 in the installed sensor and those in the light receiver 3 are optically offset in a horizontal plane, the wide angular characteristics of the optical axes are problematic in that the light receiver 3 receives the light from the light emitter 2 and operates normally; as a result, the engineer installing the sensor fails to recognize the axial offset during the matching operation and ends up with installing and fixing the area sensor in the incorrect position. If an external reflector exists in a nearby area, the above-described phenomenon will induce a dangerous situation.
FIGS. 11(a) and 11(b) illustrate how such a dangerous situation occurs. FIG. 11(a) illustrates a case of sensor installation in which the optical axes of the light emitter 2 are offset from the optical axes of the light receiver 3, as seen in a direction normal to the plane in which the optical axes lie. In FIG. 11(a), reference numeral 2 designates the light emitter, 3 designates the light receiver, 5 designates infrared beams having specified angular characteristics, 8 designates an object having a reflecting surface or a specular surfaced body such as a mirror, and the area delineated by dashed lines indicates a detectable angular range. In the case shown in FIG. 11(a), the optical axes of the light emitter 2 are offset from the optical axes of the light receiver 3 and yet the light receiver 3 receives part of the light from the emitter 2 and operates normally; as a result, the engineer has installed the area sensor in the inappropriate position believing that the matching of optical axes is satisfactory.
However, if the specular surfaced body 8 happens to exist within the detectable angular range (normal operating range) as shown in FIG. 11(b), a portion of the infrared beams 5 travels unblocked by an interrupting object 9 and is reflected by the specular surfaced body 8 as indicated by an arrow 56 so that it is eventually received by the light receiver 3 in the corresponding position. As a result, the area sensor is most likely to operate erroneously by failing to issue an alarm or shut down the machine and this can be a very dangerous situation.
An obvious solution to this problem is narrowing the angular characteristics of all optical axes in the devices of interest. However, due to structural and manufacturing variations, this approach may potentially cause the problem that the angular range over which detection can be made with the area sensor taken as a whole becomes unduly narrow. In particular, the variations that result from the use of many optical axes are unavoidably greater than in the opposite case and it is quite cumbersome to ensure that the optical axes in the light emitter are in alignment with those in the light receiver; what is more, difficulty is involved in matching the individual optical axes during sensor installation and, hence, convenience in use is not insured.
An area sensor is known that has solved the aforementioned problem. The concept of the solution is to slightly offset the individual axes in both vertical and horizontal directions so as to form a narrow infrared beam for each set of a light emitting device and a light receiving device. However, compared to the method of arranging optical axes in a straight center line, offsetting them two-dimensionally is a very cumbersome operation and increases the manufacturing cost.